A light-emitting diode (LED), offering advantages of being compact in size and power-saving, prevails in applications including illuminations, traffic lights and commercial signs. An LED is principally formed by a semiconductor material with multiple stacked epitaxial layers. For example, a blue-light LED is mainly consisted of gallium nitride-based (GaN-based) epitaxial thin films.
Referring to FIGS. 1A and 1B, a conventional vertical LED includes a sandwich structure formed by an N-type semiconductor layer 1, a light-emitting layer 2 and a P-type semiconductor layer 3. Below the P-type semiconductor layer 3, a mirror layer 4, a buffer layer 5, a binding layer 6, a silicon substrate 7 and a P-type electrode 8 are formed in sequence. A surface of the N-type semiconductor layer 1 is processed by a roughening treatment for increasing light extraction. An N-type electrode 9 is further provided. By applying a voltage to the N-type electrode 9 and the P-type electrode 8, the N-type semiconductor layer 1 is enabled to provide electrons and the P-type semiconductor layer 3 is enabled to provide holes. Light is produced by the electrons and holes combining at the light-emitting layer 2.
As previously stated, to increase light extraction, the surface of the N-type semiconductor layer 1 is processed by a roughening treatment to form an irregular surface 1A, upon which the N-type electrode 9 is directly formed. Further, the N-type electrode 9 is generally formed by a thin-film process such as sputtering or evaporation. As a result, as shown in FIG. 1B, voids 1B are formed at blind corners of the irregular surface 1A. The voids 1B not only cause poor contact that increases contact impedance, but are also likely to generate dangling bonds in molecules that limit carriers when manufacturing the irregular surface 1A. Consequently, the overall light emitting efficiency is degraded.
As disclosed by the US Publication US20100078659, an N-type metal electrode 101 is first deposited before fabricating an irregular surface on an N-type semiconductor layer 103. Thus, voids between the N-type semiconductor layer 103 and the N-type metal electrode 101 are prevented. However, as surfaces between the N-type metal electrode 101 and the N-type semiconductor layer 103 are flat surfaces, another issue of disengagement due to insufficient adhesion incurs although voids are prevented.